Agonistic interactions in female New Zealand fur seals: the functions of conspecific aggression and its implications in spatial population dynamics

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Thesis Discipline

Biological Sciences

Degree Grantor

University of Canterbury

Degree Level

Masters

Degree Name

Master of Science

Female conspecific aggression is widespread in the order Pinnipedia, which include phocids (true seals), otariids (fur seals and sea lions) and odobenid (walruses). Although the functions of female aggression have been explored in a number of pinniped species, the proposed functions vary greatly between species. The aim of this research was to investigate the functions of female aggression in the New Zealand fur seal (Arctocephalus forsteri), and to assess the vast differences in the social interactions and reproductive ecology that may explain the disparity between species. As a common social behaviour, aggression between the individuals of a group and the resulting competition for resources can have a considerable influence on the spatial population dynamics by regulating the degree of emigration and immigration. Therefore, this research also explored the effects of female aggression of the New Zealand fur seal on the dispersion of females within the rookery to quantify its effects on the spatial population dynamics, and to estimate the carrying capacity of the rookery area studied.
This study focused on a subset population of the Ohau Point seal colony, north of Kaikoura, New Zealand. A non-invasive method was used to make 184 observations of unmarked focal females over the 2014 – 2015 breeding season and the first three months of the pup rearing season. This research employed methods of quantifying aggression, such as aggression distance and the proportion of aggression, which were not commonly used in past studies of pinniped behaviour, to study its effects on the spatial population dynamics. Conspecific aggression in females was found to be prevalent in this species; however, the rate of inter-female aggression was significantly lower than other species of otariids. The results of this study showed that thermoregulation, offspring defence and resource defence were the primary functions of aggression in this species, although female aggression was found to have no influence on the dispersion of females, and subsequently, the spatial population dynamics of this species. Therefore, the nearest-neighbour distance was employed to measure the degree of female dispersion, and to predict the carrying capacity of the study area. In analysing the result of this prediction, a conclusion was reached that the nearest-neighbour distance was insufficient to represent the dispersion of females of the terrestrially-breeding colonial mammal, due to its minimal inclusion of space required between resting females for movement. This led to the proposal of a new method of measuring the individual dispersion in this species using the distance from all direct neighbours of the focal animal, and a recommendation for further research.